I used to work on these back in the 70's.There was a service manual but it was somewhat incomplete as I recall. Certainly nothing about the mod details - see below- and that's where most of the trouble was.

I will see if I can locate the guy I used to work for - Bruce Cliff - if he's still alive....You never know, he might still have the manual stashed somewhere.....

The model I knew came from the US as an RGB monitor and had a PAL front-end added locally, using NEC modules and tuner, I guess because they were readily available as spare parts..

Could you post a few pics and I'll tell you if it looks the same?

I did find this forum topic from which you might find someone who has a manual....

Back in the 90's I worked for the cat that bought everything from Advent for pennies on the dollar. He made a fortune on it. By 2000 there was next to nothing left& since only a few came in a year it went to the dump.Anyhows all the ones I worked on were either Sylvania or Electrohome chassii. Ifit uses 3 big modules its a Sylvania. Easy to fix. Just the CRT adjustments are differentbut thats not an issue unless you change one of the jugs.